Taxane compounds have received increasing attention among the scientific and medical community because of indications that various ones of these compounds, including paclitaxel (referred to in the literature as "taxol"), docetaxel (TAXOTERE.RTM.) and others, exhibit anti-tumor activity.
Paclitaxel is a naturally occurring taxane diterpenoid which is found in several species of the Yew (genus taxus, Family Taxaceae). Unfortunately, the concentration of this compound is very low. While the presence of this compound is found in the yew tree at extremely low concentrations, there are many other taxane compounds, especially 10-deacetylbaccatin III, which are able to be extracted in relatively high concentrations from renewable portions of the yew. 10-deacetylbaccatin III has the general formula: ##STR1##
In an effort to increase the available supply of the anti-tumor compounds, efforts have been made to partially synthesize the paclitaxel, docetaxel and other analogs by joining a chiral, non-racemic side chain and a protected baccatin III backbone. In some instances, it is preferable to start with baccatin III as the backbone unit while in other instances, it is possible to use 10-deacetylbaccatin III as the starting backbone unit. Baccatin III, which has the formula as follows: ##STR2## is differentiated from 10-deacetylbaccatin III by the presence of the acetate group at the C-10 location.
There have been efforts reported in the past to acylate 10-deacetylbaccatin III to provide baccatin III, but these efforts have met with mixed results. It may be observed that the 10-deacetylbaccatin III molecule has four hydroxy positions, at C-1, C-7, C-10 and C-13. A first impression from a review of this molecule would suggest that the hydroxyl positions would all be statistically acylated by an acylating compound. However, this is not true due to the steric environment of the C-1 and C-13 sites. Indeed, the hydroxy group at C-1 is so sterically encumbered that essentially no acylation would ordinarily occur at this position. Moreover, the hydroxy group at C-13 is the next most encumbered position, and it is difficult to acylate at the C-13 site. It is for this reason that the esterification of a protected baccatin III backbone with the phenylisoserine side chain, for example, has proved difficult because the C-13 hydroxy group is located within the concave region of the hemispherical taxane skeleton, thus making it difficult to access. Accordingly, attempts to acylate 10-deacetylbaccatin III results in little acylation at the C-13 position.
Reactions at the C-7 and C-10 hydroxy positions on the 10-deacetylbaccatin III molecule are quite different as these sites are dramatically more reactive than those at C-1 and C-13. Of the two sites, it has been observed that the C-7 site is more reactive. The results of attempted acylation of the 10-deacetylbaccatin III molecule using pyridine with a large excess of an acylating agent such as acetyl chloride as reported in Denis et al, "A Highly Efficient, Practical Approach to Natural Taxol", Journal of the American Chemical Society, 1988, 110, 5917. As reported in this journal article, acylation was most favored at the C-7. Acylation at C-7, of course, is highly undesirable because once acylated, it has not been demonstrated that the acetyl group at C-7 can be selectively removed thus making the compound undesirable as a precursor to any known anti-neoplastic taxane. Moreover, any selective acylation at C-10 is in extremely small quantities so as to produce a small yield.
As a result of the reactivity of the C-7 hydroxy position, attempts at converting 10-deacetylbaccatin III to baccatin III have been directed to a first step of selectively protecting the 10-deacetylbaccatin III molecule at the C-7 hydroxy position, for example, with a triethylsilyl (TES) group. This technique is reported in the Denis et al article, cited above. As described, 10-deacetylbaccatin III is converted to C-7 TES-protected 10-deacetylbaccatin III followed by the acylation of the compound at the C-10 location. Here, 10-deacetylbaccatin III is reacted with a large excess of TES-Cl and pyridine.
Alternatively, C-7 TES-protected baccatin III may be produced according to a procedure described in Kant et al "A Chemo-Selective Approach To Functionalize The C-10 Position of 10-deacetylbaccatin III Syntheses and Biological Properties of Novel C-10 Taxol.RTM. Analogs", TETRAHEDRON LETTERS, Volume 35, No. 31, TP 5543-5546 (1994). In this article, 10-deacetylbaccatin III is mixed with dimethylformamide (DMF) under a nitrogen atmosphere, and imidazole is added while stirring. TES-Cl is added dropwise followed by a quenching of the mixture. After obtaining the C-7 TES protected 10-deacetylbaccatin III, it is then acylated at C-10 using n-butyl lithium or lithium hexamethyl disilizane and acetyl chloride. The resulting C-7 TES-protected baccatin III is then deprotected at the C-7 position by any convenient method. An example of such a method uses aqueous hydrochloric acid. However, in the semi-syntheses of paclitaxel, deprotection usually is performed only after attaching the phenylisoserine side chain so that 10-deacetylbaccatin III is not converted directly into baccatin III. Previously I have reported in Method for Selective Acylation of 10-Deacetylbaccatin III, Ser. No. 08/678,759 now U.S. Pat. No. 5,750,736, a method to convert 10-deacetylbaccatin III directly to baccatin III utilizing n-butyl lithium and acetyl chloride in tetrahydrofuran at low temperature.